STUDIES OF SPECIAL LACUSTUAL HISTOKV. 99 



Lake Erie at a lower level than at present, the shore lines now forming 

 about its margin will be abandoned and another line added to the records 

 about it« borders. 



For a long i)eriod in tlie history of the Ontario l)asin, the outflowing 

 water escaped through the Mohawk valle}-. New York, as has l)een sliown 

 l)y Gilbert, and the diseharge of a large part of the Laurentian basin 

 reached tlie sea by that channel. The series of well defined Avater-marks 

 about the O.itario basin formed at this time, has been named the " Iroquois 

 beach,"' by Spencer, and the ancient lake outlined by it is known as "Lake 

 L-oquois." When the ice front retreated still fartlier northward, tlie 

 present course of the St. Lawrence was uncovered, the Mohawk channel 

 was abandoned, the water surface fell, and existing conditions were 

 established. 



During various stages in the enlargement and subsequent contraction 

 of the lakes about the southern margin of the Laurentide glacier, beaches 

 were formed which in some instances, as has been shown by Frank Lev- 

 erett, in Ohio, are continuations of the moraines deposited at the niaigin 

 of the ice where lakes did not exist in front of it. Li other instances 

 moraines oj-cur that are partially or wholly buried beneath lake sediments 

 and mark the boundaries of the ice front where it was margined by water 

 bodies. 



At many localities where the former water markings are well pre- 

 served, they Avere made on low shores, and took the form of ridges re- 

 sembling railroad embankments. The highest of these ridges marks the 

 maximum limit of the water body about Avliich it AA'as formed. As the 

 water fell the higher beaches were abandoned and others constructed at 

 levels determined by lower outlets. When the borders of the lakes were 

 of ice. shore records are wanting, but as stated above, buried monunes 

 may mark the position of the dividing line between the water and the 

 contining ice. 



Wliik- the ancient beaches were in process of construction tlic al)un- 

 dant sediments carried into the lakes, were spread out as sheets of clav 

 over the deeper portions of the basin, and at the same time the aieas near 

 shore received de})Osits of sand. Icebergs liroke away from the glaciers 

 forming the northern shores of the lakes, and floated over tlieir snrfat-es, 

 carrying stones which were drop[)ed as the ice melted, and beeame im- 

 l)edded in the clay on tlie bottom. These de])osits sniround the present 

 Laurentian lakes and underlie them. .\l)iint the liorders of Laki' Erie 

 they ai)pear as a stiff blue elay. — known to geologists as the " Erie clav," 



